Steam turbine power plants typically employ electro-hydraulic control systems which perform a variety of functions, including tripping--that is, shutting down on an emergency basis--the turbine when certain conditions arise. Such conditions include those indicating imminent damage to the turbine--for example, low bearing oil pressure, rotor overspeed, and low control fluid pressure. In addition, it may be necessary to trip the turbine as a result of dangerous conditions in other components of the power plant, such as the steam generator, the operation of which is influenced by the turbine. Typically, the presence of such dangerous conditions are determined by various sensors distributed throughout the power plant. The output from these sensors is wired into the electro-hydraulic control system which initiates the trip.
Typically, steam turbines are tripped by closing the throttle valve which controls the introduction of high pressure steam to the turbine. Since it is important to close such valves as quickly as possible upon tripping, the throttle valve is spring loaded to close. As a result, pressure must be exerted by a hydraulic fluid on the valve actuator to keep the valve open. This hydraulic pressure is maintained in a closed loop system by a pump driven by the turbine. A trip is accomplished by opening trip valves in the closed loop system which divert the hydraulic fluid to a vented drain tank, thereby dropping the pressure to the throttle valve actuator so that the spring automatically closes the throttle valve.
According to the prior art, the aforementioned trip valves are arranged in a piping system collectively referred to as a "trip control block", shown in FIG. 1. There are four trip valves 15-18, each operated by a solenoid 23, and two orifices 19 and 20 in the trip control block. The valves are arranged into two "channels" 21 and 22. Channel 21 contains valves 15 and 16 and channel 22 contains valves 17 and 18. Valves 15 and 16 and orifice 19 are arranged in parallel. In addition, valves 17 and 18 and orifice 20 are arranged in parallel. However, valves 15 and 16 and orifice 19 are arranged in series with respect to valves 17 and -8 and orifice 20. A header 10 receives hydraulic fluid from the actuator of the throttle valve so that a decrease in pressure in the header 10 results in a turbine trip. Header 10 is connected via tubing to channel 21 and orifice 19. The output from channel 21 and orifice 19 is connected via tubing to channel 22 and orifice 20.
During normal operation, all of the trip valves 15-18 remain closed so that hydraulic fluid can flow from the header 10 to a vented drain 11 only by flowing through both orifices 19 and 20. Orifices 19 and 20 are sized so that the quantity of flow they permit is well within the capability of the hydraulic fluid pump, allowing the pump and throttle valve actuator to maintain adequate pressure in the header 10 to keep the throttle valve open.
According to the prior art, the logic for tripping the turbine was hard wired. When one of the aforementioned condition sensors sensed that a turbine trip condition had been satisfied, it sent a signal via conductors 33 and 34 to relays 35 and 36, respectively. In order to avoid excessive complexity in the control system, the output of relay 35 operates both valves 15 and 16 and the output of relay 36 operates both valves 17 and 18. Thus, at a turbine trip, all four trip valves 15-18 simultaneously opened. This caused the major portion of the hydraulic fluid to flow through the trip valves 15-18 directly to the drain 11, bypassing both orifices 19 and 20. The flow coefficient of each of the trip valves 15-18 is approximately 500 times larger than that of either of the orifices 19 and 20, resulting in a very large increase in flow through the trip control block. As a result, the pressure in the header 10 from the throttle valve actuator 6 drops and the throttle valve closes, thereby tripping the turbine.
Although relays 35 and 36 direct both valves in their respective channels 21 and 22 to open simultaneously, since the valves within each channel are arranged in parallel, only one valve from each channel need be opened to cause a turbine trip. Moreover, since the channels 21 and 22 are arranged in series with respect to each other, at least one valve from each channel must be opened to cause a trip. As a result of this redundancy, the failure of any one valve in the open position will not cause an unintended turbine trip, nor will the failure of any one valve in the closed position prevent a turbine trip from being initiated. Such redundancy is important since failure to trip the turbine when appropriate could result in extensive damage to the power plant and an unintended trip of the turbine results in troublesome power disruptions.
Redundancy notwithstanding, due to the importance of proper trip valve functioning, the trip valves 15-18 are frequently tested. Since turbines are typically required to operate for long periods of time without interruption, such testing includes testing while the turbine is operating. Fortunately, since a trip will not occur unless one valve from each channel is opened, each channel can be tested separately without danger of causing an accidental trip. Accordingly, pressure switches 13 and 14 are incorporated into the trip control block downstream of orifice 19. Switch 13 closes when the pressure in the header 10 increases above a predetermined value and pressure switch 14 closes when the pressure in the header 10 decreases below a predetermined value.
According to the prior art, channel 21 is tested by actuating relay 35 and then sensing whether pressure switch 13 has closed. The opening of either valves 15 or 16 in channel 21 will cause the flow through the trip control block to bypass orifice 19 and flow through only orifice 20. As a result, the flow and, therefore, the pressure drop, across orifice 19 will decrease, thereby increasing the pressure at switch 13, causing it to close. Similarly, channel 22 is tested by actuating relay 36 and then sensing whether pressure switch 14 has closed. The opening of either valves 17 or 18 in channel 22 will cause the flow through the trip control block to bypass orifice 20 and flow through only orifice 19. As a result, the flow and; therefore, the pressure drop, across orifice 20 will decrease, thereby decreasing the pressure at switch 14, causing it to close.
Unfortunately, since, as explained above, the valve logic according to the prior art precludes operating the valves in any one channel independently, it is impossible to determine whether one valve in a channel has failed in the closed position. This is so because even if one valve in a channel has failed closed, the opening of the other valve will cause a sufficient change in pressure to actuate the pressure switch, thereby masking the failure of one valve to open. This can lead to a dangerous situation since if there has been an undetected failure of one valve in the closed position, the subsequent failure of the single previously working valve in that channel will result in an inability to trip the turbine. Unfortunately, under the hard wired logic approach of the prior art, independent operation of the valves in each channel could only be obtained by doubling the number of relays, so that each relay operated only one valve, and increasing the complexity of the logic circuits. This results in an unacceptable increase in the complexity of the control system.
Accordingly, it would be desirable to provide a system and method for testing each valve in each channel independently without introducing excessive complexity into the control system.